Several interrelated studies are underway that investigate the development of mastery motivation. Three of the projects employed the same sample of 75 children to study the interrelationships between cognition, motivation, and the environment at 6, 12, 30 months, and 6 1/2 years of age, respectively. At each age point, methods were developed that were developmentally appropriate yet conceptually similar to the data collected at the earliest age. Two separate studies were conducted to investigate the expression of mastery motivation in Down syndrome infants and to elucidate the cognitive mechanisms underlying the processing of manual actions in infancy. This last study, as well as the 6 1/2 year followup study, are currently in the data collection phase. The current focus of the 6, 12, and 30 month studies is directed toward elucidating the relationship between mastery motivation, competence, and positive and negative affect. The results indicate that affective displays are closely intertwined with aspects of parent-child interaction. Developmental changes in the expression of affect indicate that affect is a salient marker of cognitive-motivational functioning into the third year of life. with regard to mastery motivation in Down syndrome infants, the findings to date indicate that these infants display a similar distribution of behavior directed toward mastering both object-oriented and socially-oriented aspects of their environment when compared to a mental age-matched control sample. However, the relative levels of mastery behaviors are systematically depressed in the Down syndrome sample.